This invention relates to a method of detecting the presence of plateable metals in aqueous fluids, even when present at extremely low concentrations in such fluids. More particularly this invention relates to a highly sensitive method for detecting a plateable metal, such as lead, when present in trace amounts, as low as 5 parts per billion, in fluids such as drinking water, industrial effluents, body fluids and the like. For purposes of convenience, the method of the invention will be hereinafter described, for the most part, within the context of detecting lead as the plateable metal of interest, but it will be apparent from the description herein that the method of the invention is sufficiently broad in scope to enable the detection of a wide variety of other plateable metals including silver, gold, cadmium and chromium.
A continuing problem in the field of environmental protection relates to the detection of toxic metals such as lead, cadmium, arsenic, copper and the like in a wide variety of industrial effluent streams, rivers and reservoirs. While there are numerous known color spot tests for detecting lead, for example, they are generally ineffective for screening drinking water or industrial waste-water streams. This is because the concentration of lead in such fluids which is considered troublesome (often as low as 5 parts per billion) is ordinarily well below the detection limit of these spot tests; the lower limit of detection being generally about 2000 parts per billion. Thus, an initial concentration step is necessary before a conventional lead spot test can be used.
Solvent extraction with dithizone in chloroform or carbon tetrachloride is a known method of concentrating lead for spot testing. While this is considered to be a sensitive method of detection it suffers from the obvious drawback of having to use poisonous chemicals which are relatively unstable over a period of time. As a practical matter, the need to use materials such as cyanide and carbon tetrachloride precludes the general applicability of such method of detection to all but trained chemists.
Polarography and, in particular, anodic stripping voltametry (ASV) are known electrochemical methods of determining the presence of trace metals in various solutions. ASV relies on concentrating the metal of interest by cathodic deposition at an electrode (usually a hanging mercury drop electrode) under carefully controlled conditions for a known length of time. Following this concentration step, the metal is stripped from the electrode by scanning the voltage anodically to effect deplating at a voltage characteristic for the metal of interest while measuring the current-time transient. The height of the current peak corresponding to the area under the current-time curve is calibrated to provide a measure of the quantity of the metal of interest in the sample. U.S. Pat. No. 4,146,436 to Kellerman et al and U.S. Pat. No. 3,904,487 to Lieberman et al are illustrative of this electrochemical method of analysis.
While ASV is a quantative analytical technique which is highly sensitive for metal detection, it has inherent limitations for purposes of general applicability. It requires a skilled operator to carry out the method of analysis as well as expensive electrochemical equipment for effecting such polarographic technique. Consequently, ASV cannot be used by say, a layman, wishing to determine whether drinking water is free of metal contamination.
Accordingly, there is a need in the art for a highly sensitive but non-hazardous method of testing for the presence of certain plateable metals in aqueous fluids, which is semi-quantitative in nature, but nevertheless extremely useful for screening fluids such as drinking water or industrial effluent streams by relatively unskilled individuals and without the need for expensive electrochemical polarographic equipment.